This invention relates to a glass form, preferably a fiber, which resists high temperatures, at least 1900.degree. F. and higher, while retaining at least some of its tensile strength and other physical properties.
In numerous applications, fabrics are utilized in systems which resist high temperatures. An example of the use of such fabrics is in reinforced coating systems. In these systems, the fabric is embedded in a char-forming, fire-resistive coating such as those described in Deogon, U.S. Pat. No. 5,591,791. Briefly, such coatings include ablative coatings, which swell to less than twice their original thickness when exposed to fire or other thermal extremes, intumescent coatings such as those disclosed in Nielsen et al., U.S. Pat. No. 2,680,077, Kaplan, U.S. Pat. No. 3,284,216, or Ward et al., U.S. Pat. No. 4,529,467, which swell to produce a char more than five times the original thickness of the coating, and subliming char-forming coatings of the type disclosed in Feldman, U.S. Pat. No. 3,849,178, which undergo an endothermic phase change and expand two to five times their original thickness to form a continuous porosity matrix. The intumescent and subliming coatings are denoted "active" thermal protective coatings.
The time required for a given temperature rise across a predetermined thickness of the composition, under specified heat flux, environmental, and temperature conditions, is a measure of the composition's effectiveness in providing thermal protection to an underlying substrate.
Eventually, the char is consumed by physical erosion and by chemical processes, such as oxidation by oxygen in the air and by free radicals produced by the coating or otherwise in a fire environment, and protection is substantially reduced. Before the char is totally consumed, degradation of the char layer leaves it crumbled and without the necessary strength to sustain itself, causing it to fail by being blown off or simply falling off (spalling).
Some of these chars degrade rapidly during exposure to high temperature, high heat flux environments. In the case of coatings which swell when exposed to thermal extremes, the degradations are usually in the form of fissures which are formed in the char as a result of differential thermal stresses produced by the high thermal gradients with them, and differential thermal expansion between the virgin material and the char.
To increase the strength of char layers during exposure to thermal extremes, and to limit spalling and fissures, fabrics have long been incorporated in the coating materials. As set out in Feldman et al., U.S. Pat. No. 5,622,774, fiberglass fabric provides an inexpensive, easy to install, reinforcement in many high temperature applications. In certain applications, however, such as coatings which may be exposed to high velocity petroleum fires or to high-temperature, high heat flux fires which will raise the fabric to temperatures above the softening point of the glass (around 1600.degree. F.), the fiberglass fabric has disintegrated. Other fabrics have therefore been required. Graphite cloth, as taught in the foregoing Feldman et al. U.S. Pat. No. 5,622,774 and in Kobayashi et al., U.S. Pat. No. 5,401,793, is very expensive. Refractory materials, such as quartz (Refrasil) fabric is also expensive. Metal mesh is inexpensive but it is heavy and difficult to install, particularly because it generally requires welding metal studs to the substrate to be protected.
Other examples of fabric-reinforced systems are laminates in which the fabric is embedded directly in a structural resin material itself, for example in the structure of a furnace or a rocket nozzle. Generally, these materials also produce a char when exposed to sufficiently high temperatures, although in many applications they are routinely exposed to high temperatures below their char-forming temperature for extended periods. In other applications they are exposed for short periods to temperature, heat flux, and environmental conditions which do not cause a char to form, but which are sufficiently high to cause serious loss of structural properties. Examples of these latter systems are automobile gasoline tanks and trunks, which can be made of plastic material if they can pass a test involving preventing structural failure (such as drop through or explosion in the case of a gasoline tank) when the tank or trunk is placed over a fire of a specified temperature and intensity for a predetermined period such as two minutes. In all of these conditions, a fabric which resists complete degradation under the foregoing conditions can provide sufficient structural integrity to impede failure of the system.
Attempts have been made for many years to produce a glass fiber which retains a substantial portion of its mechanical properties even when subjected to very high temperatures, greater than 1600.degree. F. (871.degree. C.), and preferably on the order of 1900.degree. to 2000.degree. F. (1038.degree. to 1093.degree. C.). Examples are Nordberg, U.S. Pat. No. 2,461,841, Parker et al., U.S. Pat. No. 2,491,761, and Leeg et al., U.S. Pat. No. 2,992,960. These patents all involve leaching of the glass fiber with mineral acid, followed by treatment with a sizing material. Heretofore, such attempts have failed to provide a reliable, reproducible, and efficient process of converting commercial grade fiberglass (such as Type E and Type F glass fibers) into a material capable of withstanding elevated temperatures and aerodynamic shear which may be coupled with elevated temperatures.